The goal of the proposed research is to understand specific ionic and molecular mechanisms that act at the cellular level in the vertebrate retina during light- and dark-adaptation. Photic stimulation of the retina initiates a process by which the rods remove potassium from the extracellular space, leading to a decrease in extracellular potassium concentration (K+)o. This decrease in (K+)o is known to affect the retinal pigment epithelium (RPE), and it may also affect certain functions of the rods themselves. With prolonged illumination, the changes in (K+)o are complex, showing partial recovery during illumination and an overshoot at termination of illumination. These movements of K+ may be caused by changes in the rate of the Na+/K+ pump in the rod membrane, by changes in membrane conductance, or by diffusion, and are likely to be accompanied by movements of sodium and chloride. These events will be investigated electrophysiologically in the retina of the toad, Bufo marinus, by intracellular measurements of rod membrane voltage, combined with intra- and extracellular measurements of ionic concentrations using ion-selective microelectrodes. Following prolonged or intense light adaptation, rods in the isolated retina (where the RPE is absent) never regain their dark-adapted response characteristics. Superfusion with an hydrolysis-resistant analog of GTP causes recovery to occur, however, and thus a GTP-binding protein has been implicated in the control of rod responsiveness. The existence of this protein will be investigated further by intracellular recording of rod photoresponses both during superfusion with additional hydrolysis-resistant analogs of GTP and other substances that activate GTP-binding proteins, and during intracellular injections of these compounds. The proposed experiments will provide basic knowledge of mechanisms that function at the membrane level in rods during light- and dark-adaptation, and how these mechanisms affect, or are affected by, the RPE. Knowledge of these basic mechanisms, and of their dependence upon the normal association between the rods and the RPE, may lead to an increased understanding of disease processes that disrupt function in the outer retina.